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More than 80% of traumatic brain injury (TBI) patients suffer from mild TBI (mTBI). However, even mTBI carries the risk of late pituitary dysfunction. A predictive biomarker at the time of injury that could identify patients who subsequently may develop permanent pituitary dysfunction would help to direct patients toward endocrine care. We enrolled 508 TBI patients (406 with mTBI) into our study. Blood samples were collected for identification of predictive biomarkers of late pituitary dysfunction at the time of admission. Follow-up blood samples were collected between 6 and 12 months after the TBI and were evaluated for pituitary function. Of the 406 mTBI patients, 76 were available for follow-up. Pre-existing mild pituitary dysfunction was found for 15 patients based on hormone levels at the time of injury. Of the remaining 61 patients, 10 have shown deficiency in at least one pituitary hormone: 4 had growth hormone deficiency, 3 gonadotropin, 2 thyrotropin, and 1 patient combined gonadotropin and thyrotropin deficiency. Hence, newly developed pituitary hormone deficiency was found in 16% of mTBI patients. Neither the cause of mTBI nor its complications were predictive of late pituitary dysfunction. Of the hemostasis parameters studied, lower plasminogen activator inhibitor type 1 (PAI-1) level at the time of injury was found to be predictive for the development of late pituitary dysfunction; sensitivity, specificity, and positive and negative predictive values were 80%, 67%, 32%, and 94%, respectively. Even mTBI carries a substantial risk of endocrine consequences. Serum PAI-1 level at the time of TBI may serve as a predictive biomarker of late pituitary dysfunction in mTBI patients.